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United States Patent |
5,660,781
|
Moriya
,   et al.
|
August 26, 1997
|
Process for preparing glass ceramic green sheets
Abstract
Low-firing glass ceramic green sheets useful in the production of glass
ceramic multilayer substrates are prepared from a coarse raw powder
material comprising a B.sub.2 O.sub.3 -containing glass powder. The coarse
raw powder material is initially subjected to wet grinding, either in an
alcohol-free organic solvent in the presence or absence of an organic
binder, or in an alcohol-containing organic solvent in the presence of an
organic binder, until the powder is comminuted to a particle size suitable
for tape casting. The wet-ground powder is slurried with an organic
solvent and an organic binder, and the slurry is cast into sheets. The
resulting green sheets have improved elongation and can be punched to form
fine through holes with a small pitch.
Inventors:
|
Moriya; Yoichi (Nishinomiya, JP);
Yamade; Yoshiaki (Nishinomiya, JP);
Uno; Koichi (Nagoya, JP)
|
Assignee:
|
Sumitomo Metal Industries, Ltd. (Osaka, JP);
Sumitomo Metal Ceramics, Inc. (Mine, JP)
|
Appl. No.:
|
493996 |
Filed:
|
June 23, 1995 |
Foreign Application Priority Data
Current U.S. Class: |
264/212; 264/144; 264/650 |
Intern'l Class: |
B29D 007/00; C04B 035/64 |
Field of Search: |
156/89
264/63,144,212
|
References Cited
U.S. Patent Documents
4621066 | Nov., 1986 | Nishigaki et al. | 501/128.
|
4833104 | May., 1989 | MacDowell et al. | 501/10.
|
4833497 | May., 1989 | Claar et al. | 29/623.
|
5306646 | Apr., 1994 | Lauf | 437/2.
|
5514451 | May., 1996 | Kumar et al. | 428/210.
|
Foreign Patent Documents |
59-195573 | Nov., 1984 | JP.
| |
Primary Examiner: Czaja; Donald E.
Assistant Examiner: Ruller; Jacqueline A.
Attorney, Agent or Firm: Burns, Doane, Swecker & Mathis, LLP
Claims
What is claimed is:
1. A process for preparing boron-containing glass ceramic green sheets
comprising the steps of:
forming a wet ground powder by wet-grinding a coarse raw powder material
which comprises a B.sub.2 O.sub.3 -containing glass powder in an
alcohol-free organic solvent in the presence or absence of an organic
binder until the particle size is sufficiently reduced to be suitable for
tape casting,
forming a slurry in which the wet-ground powder is dispersed in an organic
solvent containing at least an organic binder,
casting the slurry into sheets, and drying the resulting wet sheets.
2. A process for preparing boron-containing glass ceramic green sheets
comprising the steps of:
forming a wet ground powder by wet-grinding a coarse raw powder material
which comprises a B.sub.2 O.sub.3 -containing glass powder in an
alcohol-containing organic solvent in the presence of an organic binder
until the particle size is sufficiently reduced to be suitable for tape
casting,
forming a slurry in which the wet-ground powder is dispersed in an organic
solvent containing at least an organic binder,
casting the slurry into sheets, and
drying the resulting wet sheets.
3. The process of claim 2 wherein the organic binder is an acrylic resin
and the proportion of the alcohol in the organic solvent is not greater
than 50% by weight.
4. The process of claim 2 wherein the organic binder is a butyral resin and
the proportion of the alcohol in the organic solvent is not greater than
80% by weight.
5. The process of claim 1 wherein the B.sub.2 O.sub.3 -containing glass
powder is selected from the group consisting of borosilicate-based glass
powders, MgO-Al.sub.2 O.sub.3 -SiO.sub.2 -B.sub.2 O.sub.3 -based glass
powders, and CaO-Al.sub.2 O.sub.3 -SiO.sub.2 -B.sub.2 O.sub.3 -based glass
powders.
6. The process of claim 2 wherein the B.sub.2 O.sub.3 -containing glass
powder is selected from the group consisting of borosilicate-based glass
powders, MgO-Al.sub.2 O.sub.3 -SiO.sub.2 -B.sub.2 O.sub.3 -based glass
powders, and CaO-Al.sub.2 O.sub.3 -SiO.sub.2 -B.sub.2 O.sub.3 -based glass
powders.
7. The process of claim 1 wherein the coarse raw powder material further
comprises a ceramic powder as a filler.
8. The process of claim 2 wherein the coarse raw powder material further
comprises a ceramic powder as a filler.
9. The process of claim 1 wherein the wet grinding is performed by ball
milling.
10. The process of claim 2 wherein the wet grinding is performed by ball
milling.
11. The process of claim 1 wherein a plasticizer is further present in the
organic solvent used in wet grinding.
12. The process of claim 2 wherein a plasticizer is further present in the
organic solvent used in wet grinding.
13. The process of claim 1 wherein a dispersant is further present in the
organic solvent used in wet grinding.
14. The process of claim 2 wherein a dispersant is further present in the
organic solvent used in wet grinding.
15. The process of claim 1 wherein a plasticizer and a dispersant are
further present in the organic solvent used in wet grinding.
16. The process of claim 2 wherein a plasticizer and a dispersant are
further present in the organic solvent used in wet grinding.
17. The process of claim 1 wherein the wet grinding is continued until the
powders are comminuted to an average particle size in the range of from
about 1 to about 5 .mu.m.
18. The process of claim 2 wherein the wet grinding is continued until the
powders are comminuted to an average particle size in the range of from
about 1 to about 5 .mu.m.
19. The process of claim 1 wherein during the wet grinding the coarse raw
powder material is wet ground in a non-aqueous medium, the glass powder
contains 5 to 30 weight % B.sub.2 O.sub.3 or the glass powder is present
in an mount of 30 to 90 weight % of the coarse raw powder.
20. The process of claim 2 wherein during the wet Finding the coarse raw
powder material is wet ground in a non-aqueous medium, the glass powder
contains 5 to 30 weight % B.sub.2 O.sub.3 or the glass powder is present
in an mount of 30 to 90 weight % of the coarse raw powder.
21. The process of claim 1 wherein the organic binder covers essentially
all surfaces of the B.sub.2 O.sub.3 -containing glass powder.
22. The process of claim 2 wherein the organic binder covers essentially
all surfaces of the B.sub.2 O.sub.3 -containing glass powder, the organic
binder protecting B.sub.2 O.sub.3 -rich phases appearing on surfaces of
the glass powder from reacting with alcohol in the alcohol-containing
organic solvent.
Description
BACKGROUND OF THE INVENTION
The present invention relates to a process for preparing glass ceramic
green sheets for use in the production of substrates in glass ceramic
printed circuit boards on which electronic devices such as LSI's are
mounted.
In recent years, plastic substrates, such as those made of a phenolic resin
or glass-reinforced epoxy resin, which have predominantly been used as
substrates in conventional printed circuit boards, have rapidly been
replaced by ceramic substrates, particularly ceramic multilayer substrates
due to the demand for increased mounting density and improved reliability
of printed circuit boards.
Ceramic multilayer substrates are usually produced either by the multilayer
printing process or the green sheet laminating process. In the multilayer
printing process, a conductive paste and an insulating paste are
alternately applied in predetermined patterns onto a sintered or green
ceramic sheet, which is then fired. The green sheet laminating process
comprises laminating a plurality of green sheets, each having a desired
circuit pattern printed thereon with a conductive paste and through holes
(also called via holes) filled with a conductive paste, and co-firing the
resulting laminate to sinter the green sheets and the conductive paste
simultaneously. Of these, the green sheet laminating process is prevalent
because a larger number of layers can be readily laminated with more
precise circuit patterns.
As a ceramic material for ceramic substrates, alumina (Al.sub.2 O.sub.3)
has primarily been employed in view of its good electrical insulating
properties and heat resistance and its relatively low material costs.
Since alumina-based ceramics are sintered at a temperature as high as
1550.degree. C., it is necessary to use a conductive paste containing a
conductive powder of a refracfory metal, e.g, W (tungsten) or Mo
(molybdenum) or a mixture thereof, to print alumina-based green sheets or
fill through holes formed therein such that the conductive powder
withstands the high temperature during firing without a significant loss
of conductivity.
However, the electric resistivity of W and Mo is relatively high for a
metal. Therefore, as circuits become finer and thinner to increase the
mounting density, the resistance of circuits made of W or Mo is
appreciably increased to such a degree that it may interfere with the
function of the circuit by an increased signal delay.
In order to eliminate the problem just described which has been encountered
by the use of W or Mo powder in a conductive paste, multilayer substrates
of low-firing ceramic materials have been developed. These ceramic
materials can be sintered at a temperature below 1000.degree. C., and
therefore it is possible to fire them along with a non-refractory,
low-resistivity conductive metal such as Ag or Cu in a conductive paste to
form substrates.
One class of low-firing ceramic substrates is a glass ceramic substrate.
Glass ceramic substrates are attracting much attention for the reason that
they have insulating properties and heat resistance comparable to
conventional alumina substrates, a dielectric constant lower than alumina
(leading to a reduced signal delay) and a thermal expansion coefficient
close to that of silicon (thereby facilitating mounting of flip chips).
The raw material for typical glass ceramic substrates is a combination of
a boron-containing glass powder such as a powder of a borosilicate-based
glass, MgO-Al.sub.2 O.sub.3 -SiO.sub.2 -B.sub.2 O.sub.3 -based glass, or
CaO-Al.sub.2 O.sub.3 -SiO.sub.2 -B.sub.2 O.sub.3 -based glass, with a
ceramic powder such as an alumina powder which serves as a filler.
Glass ceramic multilayer substrates are generally prepared as follows. A
glass powder and a ceramic powder are subjected to wet grinding together
in a ball mill until the particle size is reduced to a level suitable for
use in tape casting into green sheets (e.g., 1 to 5 .mu.m in average
particle diameter), thereby achieving Both size reduction and thorough
mixing of the glass and ceramic powders. Commercially available glass
powders and ceramic powders are normally coarse and have an average
particle diameter in the range of about 5 .mu.m to about 100 .mu.m.
Therefore, it is necessary to reduce the particle size of the powders in
the first step. The size reduction or comminution has normally been
performed by wet grinding in a ball mill using water as a liquid medium.
Subsequently, the ground powder mixture is recovered and dried to remove
water. In most cases the dried powder mixture is agglomerated. Therefore,
the powder mixture is disintegrated before it is mixed with an organic
solvent as a dispersing medium, an organic binder, and other additives
such as a dispersant and plasticizer to form a slurry called a "slip". The
slurry is then cast into sheets by a suitable method, typically by the
tape casting method using a doctor blade, and glass ceramic green sheets
are obtained after the sheets are dried to remove most of the solvent.
The resulting glass ceramic green sheets are punched to form through holes,
if necessary, and a conductive paste is then printed onto each sheet to
form a circuit pattern thereon and fill the through holes, if present. A
plurality of such green sheets are laminated, and the resulting laminate
is co-fired at a temperature below 1000.degree. C. (usually between
900.degree. C. and 1000.degree. C.) to sinter the conductive paste and
green sheets simultaneously to give a glass ceramic multilayer substrate.
The firing step is usually preceded by a degreasing step whereby organics
are removed from the laminate by heating to a temperature substantially
lower than the firing temperature.
In the preparation of alumina-based ceramic green sheets which are fired at
a higher temperature, it is known that a fine alumina powder which has
been comminuted is processed in a wet ball mill, for purposes of
disintegration and mixing with additives such as an organic binder, using
an organic solvent as a liquid medium. See, Japanese Patent Application
Kokai No. 59-195573(1984). However, such wet ball milling is not intended
for comminution or substantial size reduction of the alumina powder.
Whether the green sheets to be prepared are of a low-firing glass ceramic
or a conventional high-firing ceramic such as alumina, wet ball milling in
water has been employed in the grinding step to reduce the particle size
of a raw material to a level suitable for tape casting.
In order to meet the recent demand for still higher integration of LSI's
and further size reduction of printed circuit boards, through holes formed
in green sheets are required to have a smaller diameter and a smaller
pitch.
In general, when a ceramic green sheet is punched to form through holes
with a smaller diameter and pitch, it is important to prevent as much as
possible the formation of burrs around each hole and the formation of
cracks between adjacent holes. For this purpose, it is advantageous that
the ceramic green sheet have an increased elongation.
A ceramic green sheet is assured to have an increased elongation if the
surfaces of the ceramic powder particles present in the sheet are
completely covered with an organic binder, thereby improving the adhesion
between ceramic powder particles.
In the prior art glass ceramic green sheets prepared in the above-described
manner, however, it was frequently found that an increased elongation
could not be achieved due to failure to completely cover the surfaces of
the powder particles unless an excessively large amount of the organic
binder is added. The use of the organic binder in a significantly
increased amount not only adds to the manufacturing costs of the green
sheets but also may increase the amount of residual carbon remaining after
firing, which causes the resulting sintered substrate to turn gray and
have a decreased insulation resistance.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a process for preparing
glass ceramic green sheets which are assured to have an increased
elongation in order to enable the sheets to be punched to form fine
through holes with a small pitch using an organic binder in a limited
amount.
The present invention provides a process for preparing boron-containing
glass ceramic green sheets comprising the steps of: subjecting a coarse
raw powder material which comprises a B.sub.2 O.sub.3 -containing glass
powder to wet grinding until the particle size is sufficiently reduced to
be suitable for tape casting; forming a slurry in which the wet-ground
powder is dispersed in an organic solvent containing at least an organic
binder; casting the slurry into sheets; and drying the resulting sheets.
In one aspect of the present invention, the wet grinding step is performed
in an alcohol-free organic solvent in the presence or absence of an
organic binder. In another aspect of the present invention, the wet
grinding step is performed in an alcohol-containing organic solvent in the
presence of an organic binder.
DETAILED DESCRIPTION OF THE INVENTION
The present inventors investigated why an increased elongation cannot be
attained unless the amount of an organic binder added is significantly
increased and found that some ingredient of the raw glass powder is
dissolved out during wet grinding in water, thereby rendering the
wet-ground glass powder porous and increasing the surface area thereof
significantly. If the surface area is increased with a given particle
size, an increased amount of an organic binder must be used to completely
cover the surfaces of the powder particles.
The reason why the wet-ground glass powder becomes porous is thought to be
as follows. As described above, a glass ceramic substrate is normally
prepared from a boron-containing glass powder such as a borosilicate-based
glass powder, MgO-Al.sub.2 O.sub.3 -SiO.sub.2 -B.sub.2 O.sub.3 -based
glass powder, or CaO-Al.sub.2 O.sub.3 -SiO.sub.2 -B.sub.2 O.sub.3 -based
glass powder. The incorporation of boron in the form of B.sub.2 O.sub.3 in
the glass powder is essential for a low-firing glass ceramic green sheet
in order to decrease the softening point of the glass so that the green
sheet can be fired at a low temperature below 1000.degree. C. Although
B.sub.2 O.sub.3 itself is soluble in water, B.sub.2 O.sub.3 in a glass
formed by melting B.sub.2 O.sub.3 along with other one or more ingredients
is much less soluble in water if it is uniformly distributed in the glass.
However, the distribution of B.sub.2 O.sub.3 in a glass is, in fact, not
ideally uniform and there exist separate B.sub.2 O.sub.3 -rich phases in
the glass. B.sub.2 O.sub.3 present in these phases of a glass powder is
soluble in water to a certain degree and can be dissolved in water while
the glass powder is being processed by wet grinding in water. Furthermore,
as fresh surfaces are formed by grinding, they are immediately brought
into contact with water, and B.sub.2 O.sub.3 present in the B.sub.2
O.sub.3 -rich phases appearing on the fresh surfaces is dissolved in
water. Repeated occurrence of these phenomena results in the formation of
pores in the wet-ground glass powder, thereby significantly increasing the
specific surface area of the powder.
According to the present invention, the coarse raw powder material
comprising a B.sub.2 O.sub.3 -containing glass powder is processed by wet
grinding in an organic solvent, i.e., in the absence of water, thereby
preventing the formation of pores which lead to a significant increase in
surface area, and making it possible to prepare green sheets having an
increased elongation without using an increased amount of an organic
binder. When the organic solvent used in wet grinding is free from an
alcohol, wet grinding may be performed in the presence of an organic
binder but preferably in the absence of an organic binder. When the
solvent is comprised partially or solely of an alcohol, wet grinding
should be performed in the presence of an organic binder.
The coarse raw powder material used in the preparation of glass ceramic
green sheets comprises a B.sub.2 O.sub.3 -containing glass powder, which
can be selected from borosilicate-based glass powders, MgO-Al.sub.2
O.sub.3 -SiO.sub.2 -B.sub.2 O.sub.3 -based glass powders, and CaO-Al.sub.2
O.sub.3 -SiO.sub.2 -B.sub.2 O.sub.3 -based glass powders. These B.sub.2
O.sub.3 -containing glass powders may further contain additional one or
more metal oxides such as an alkali metal oxide in a minor amount. A
B.sub.2 O.sub.3 -free glass powder may be used in combination with the
B.sub.2 O.sub.3 -containing glass powder. The amount of B.sub.2 O.sub.3 is
usually in the range of about 5% to about 30% based on the total weight of
the glass powder.
In addition to the B.sub.2 O.sub.3 -containing glass powder, a ceramic
powder is usually present as a filler in the raw powder material. The
ceramic powder useful as a filler is typically an alumina powder, but one
or more powders selected from cordierite, aluminum nitride, quartz,
mullite, and the like may be used in place of or in combination with
alumina.
The proportions of glass and ceramic powders in the raw powder material are
not critical, but the weight ratio of glass to ceramic powder is usually
in the range of about 90:10 to about 30:70 and preferably about 80:20 to
about 50:50.
As described previously, since commercially available glass and ceramic
powders are normally too coarse to be used for tape casting to form green
sheets, the raw powder material, which is usually a combination of a glass
powder and a ceramic powder, is initially subjected to wet grinding so as
to reduce the particle size sufficiently to be suitable for tape casting
and thoroughly mix the glass and ceramic powders. In accordance with the
present invention, such wet grinding is performed using an organic solvent
rather than water as a liquid medium.
Any organic solvent can be used as a liquid medium in wet grinding as long
as it does not have an appreciable adverse effect on the glass and ceramic
powders. Useful organic solvents include aromatic hydrocarbons such as
xylene, toluene, and ethylbenzene ketones such as methyl ethyl ketone and
diethyl ketone; esters such as ethyl acetate, isopropyl acetate, and butyl
acetate; and alcohols such as ethanol, propanol, isopropanol, butanol,
pentanol, and hexanol. One or more of these solvents may be used.
Of these solvents, an alcohol may react with B.sub.2 O.sub.3 -rich phases
present in the B.sub.2 O.sub.3 -containing glass powder to cause
dissolution of B.sub.2 O.sub.3 into the solvent. The dissolved B.sub.2
O.sub.3 tends to combine with the alcohol to form an organoboron compound.
Since the bonding strength between the boron and alcohol is high, the
alcohol combined with B.sub.2 O.sub.3 cannot be completely evaporated
during firing of green sheets, thereby increasing the amount of residual
carbon in the sintered substrate, which, in turn, causes the substrate to
turn gray or have a decreased insulation resistance.
In order to eliminate this problem, when the organic solvent used in wet
grinding contains an alcohol, an organic binder is added to the organic
solvent to dissolve therein and wet grinding is performed in the presence
of the organic binder. The surfaces of the glass powder particles are
covered with an organic binder and protected in such a manner that the
B.sub.2 O.sub.3 -rich phases appearing on the surfaces of the glass powder
particles are prevented from reacting with the alcohol and being dissolved
in the solvent. Fresh surfaces formed by grinding are also protected by
the organic binder in the same manner as above immediately upon formation
thereof. As a result, after the firing step, the resulting sintered
substrate is prevented from having an increased residual carbon content
and hence turning gray and having a decreased insulation resistance. A
plasticizer and/or a dispersant may be added to the organic solvent along
with the organic binder.
When an alcohol is present in the organic solvent used for wet grinding,
the solvent may consist essentially of the alcohol. However, since most
organic binders do not have a high solubility in an alcohol, it is
preferable that the solvent be a mixed solvent of an alcohol and a
non-alcoholic organic solvent. In such cases, the proportions of the
alcoholic and non-alcoholic solvents should be selected such that the
resulting mixed solvent can dissolve a sufficient amount of the organic
binder used in the preparation of green sheets. For example, when the
organic binder is an acrylic resin, it is preferable that the proportion
of an alcohol in the mixed solvent be 50% by weight or less. In the case
of a butyral resin as an organic binder, the proportion of an alcohol is
preferably 80% by weight or less.
When the solvent used in wet grinding does not contain an alcohol, it is
preferable to perform wet grinding in the absence of an organic binder in
view of efficiency of comminution, although an organic binder or a
combination of an organic binder and a plasticizer and/or dispersant may
be added to the alcohol-free solvent.
The organic solvent is used in wet grinding in an amount sufficient to
provide smooth grinding, usually in the range of from about 30 to about
150 parts and preferably from about 30 to 100 parts by weight for each 100
parts by weight of the coarse raw powder material to be ground.
After wet grinding, it is not necessary to remove the organic solvent prior
to the preparation of a slurry to be cast into sheets, but the organic
solvent can remain so as to constitute part or all of the dispersing
medium of the slurry. Therefore, it is not advantageous from a cost
viewpoint to increase the amount of the organic solvent used in wet
grinding so as to be much larger than the amount used in the preparation
of the slurry.
When wet grounding is performed in the presence of an organic binder of an
organic binder and a plasticizer and/or dispersant, it is preferable to
add the organic binder in an amount of from about 0.5 to about 10 parts
and more preferably from about 0.5 to about 5.0 parts, the plasticizer in
an amount of from about 0.1 to about 4.0 parts and more preferably from
about 0.5 to about 3.0 parts, and the dispersant in an amount of from
about 0.1 to about 3.0 parts and preferably from about 0.5 parts to 2.0
parts, for each 100 parts of the coarse raw powder material to be ground.
The organic binder and plasticizer may be selected from those
conventionally used in the preparation of glass ceramic green sheets.
Useful organic binders are organic resins including acrylic resins,
butyral resins, vinyl acetate copolymers, polyvinyl alcohols, and vinyl
chloride resins. Preferably the organic binder is selected from acrylic
resins and butyral resins. The plasticizer is preferably selected from
phthalate esters such as dioctyl phthalate (DOP) and dibutyl phthalate
(DBP), but a polyalkylene glycol such as triethylene glycol may be used as
a plasticizer. The dispersant is preferably selected from acrylic acid
oligomers and sorbitan monooleate.
Wet grinding of the coarse raw powder material is conveniently performed in
a ball mill, but a different type of wet grinding mill such as an
attrition mill or bead mill may be used.
In the practice of wet grinding in a ball mill, a glass powder and a
ceramic powder both having a coarse particle size may be placed into the
ball mill in a predetermined proportion, and an organic solvent and
optionally an organic binder and/or a plasticizer are added in amounts in
the above-described ranges. Subsequently, ceramic bails made of alumina,
for example, are added in an amount of from about 200 to about 500 parts
by weight for each 100 parts of the total weight of the coarse powders,
and wet ball milling is continued until the powders are comminuted to an
average particle size in the range of from about 1 to about 5 .mu.m and
preferably from 1.5 to 3.5 .mu.m, for example. Generally about 10 to 100
hours of ball milling are required.
The wet-ground powder mixture is then used to form a slurry (slip) which
comprises the powder mixture dispersed in an organic solvent containing an
organic binder and optionally a plasticizer and a dispersant. The slurry
usually contains, on a weight basis, from about 30 to about 150 parts and
preferably from about 30 parts to 100 parts of an organic solvent, from
about 5 to about 30 parts and preferably from about 10 parts to 25 parts
of an organic binder, from about 0.1 to about 4.0 parts and preferably
from about 0.5 parts to 3.0 parts of a plasticizer, and from about 0.1 to
about 3.0 parts and preferably from about 0.5 parts to 2.0 parts of a
dispersant for each 100 parts of the powder mixture of glass and ceramic
powders.
It is advantageous that the organic solvent used in wet grinding and any
ingredient (organic binder, plasticizer, and/or dispersant) Added to the
solvent for wet grinding be used so as to remain in the slurry. Thus, the
powder-containing mixture obtained by removing the ceramic balls may be
used as is for the preparation of the slurry. If necessary, the amount of
the organic solvent may be adjusted by evaporation or supplementation and
an organic binder, a plasticizer, and/or a dispersant may be added to the
mixture along with a dispersant. The organic binder, plasticizer, and
dispersant used in this step may be selected from the above-described
groups.
The resulting slurry is cast into sheets by a conventional manner, usually
by tape casting using a doctor blade, and the wet sheets thus-obtained are
then dried either at room temperature or at an elevated temperature until
a substantial part of the solvent is removed.
In a conventional wet grinding process in water, since the ground powder
mixture of glass and ceramic powders is once dried prior to use in the
preparation of a slurry, it is necessary to subject the ground powder to
disintegration for a prolonged period of time. In accordance with the
present invention, however, such a time-consuming disintegration step can
be eliminated since the ground powder mixture can be directly used to
prepare a slurry without drying.
More importantly, in accordance with the present invention, the increase in
specific surface area of the powder mixture during wet grinding can be
minimized since the formation of pores in the glass powder due to
dissolution of B.sub.2 O.sub.3 can be effectively prevented. As a result,
when the ground powder mixture has an average particle diameter of from
1.5 to 3.5 .mu.m, for example, it will have a specific surface area
between about 1.5 and about 5.0 m.sup.2 /g. In contrast, if wet grinding
is conducted in water in a conventional manner until the same average
particle diameter is reached, the resulting ground powder mixture will
have a specific surface area of about 5 to 10 times as large as the
above-described level.
As a result, in order to prepare glass ceramic green sheets having good
elongation from a powder mixture which has been wet ground in a
conventional manner, it is necessary to add an increased amount of an
organic binder to the slurry used to cast into green sheets. On the
contrary, with a powder mixture which has been wet ground in an organic
solvent in accordance with the present invention, glass ceramic green
sheets having good elongation can be formed from a slurry containing a
limited amount of an organic binder since the powder mixture is prevented
from having an excessively increased surface area, and each particle can
be completely covered with such a limited amount of the organic binder.
The good elongation of the glass ceramic green sheets makes it possible to
punch them so as to form through holes with a small diameter and a small
pitch without the formation of burrs around the holes and cracks between
the holes, thereby facilitating an increase in the degree of integration
of LSI's and a decrease in the size of printed circuit boards.
Furthermore, the use of a limited amount of the organic binder decreases
the manufacturing costs of glass ceramics substrates and minimizes the
amount of residual carbon in the substrates after firing, thereby
preventing the sintered substrates from turning gray and having a
decreased insulation resistance.
The glass ceramic green sheets prepared in accordance with the present
invention are particularly suitable for use in the production of glass
ceramic multilayer substrates by the green sheet laminating process, but
they can be used in the production of glass ceramic multilayer substrates
by the multilayer printing process on an unfired (green) substrate sheet
or in the production of single layer substrates.
The following examples are presented to further illustrate the present
invention. These examples are to be considered in all respects as
illustrative and not restrictive. In the examples, all percents and parts
are by weight unless otherwise indicated.
EXAMPLES
Example 1
The coarse powder material used in this example consisted essentially of an
MgO-Al.sub.2 O.sub.3 -SiO.sub.2 -B.sub.2 O.sub.3 -R.sub.2 O glass powder
(R: alkali metal, B.sub.2 O.sub.3 content: 18%, average particle diameter:
12 .mu.m), an alumina powder (average particle diameter: 10 .mu.m), and a
cordierite powder (average particle diameter: 10 .mu.m), all of which were
commercially available.
A ball mill was charged with 100 parts of the glass powder, 25 parts of the
alumina powder, and 7 parts of the cordierite powder, and an organic
solvent or mixed organic solvent indicated in Table 1 was added to the
ball mill in an amount between 50 and 150 parts, the amount being selected
so as to be equivalent to the volume of water having the same weight as
the total weight of the powders (=132 parts). Thereafter, 400 parts of
alumina balls (15 mm in diameter) were added and wet grinding was
performed for about 48 hours until the powder mixture had an average
particle diameter of 2 .mu.m.
After the alumina balls were removed from the resulting mixture, the
mixture was dried by heating to completely evaporate the organic solvent
in order to determine the specific surface area of the ground powder
mixture by the BET adsorption method. The measurement was performed on a
Microtix Acusorp Model 2100 (Shimadzu) using a Kr gas for adsorption after
deaeration for 60 minutes at 200.degree. C.
To 100 parts of the dried wet-ground powder mixture, 40 parts of xylene as
a dispersing medium, 2 parts of DOP (dioctyl phthalate) as a plasticizer,
17 parts of an acrylic resin (polymethyl methacrylate) as an organic
binder, and 1 part of an acrylic acid oligomer as a dispersant were added,
and the mixture was thoroughly dispersed for 12 hours to form a slurry.
The slurry was cast into sheets using a doctor blade, and the wet sheets
were dried at 80.degree. C. to form 200 .mu.m-thick green sheets.
The resulting glass ceramic green sheets were evaluated in terms of tensile
strength and elongation at break, and punchability as follows.
The tensile test to determine the tensile strength and elongation at break
was performed on a tensile testing machine (equipped with a load cell of 5
kg) at a rate of pulling of 20 mm/min using samples measuring 10
mm.times.20 mm.
The punchability was measured by forming 400 holes (20 holes by 20 holes)
in a square sample using an NC punching machine. The diameter of each hole
was 3.5 mm and the pitch between two adjacent holes was 4 mm. The punched
green sheet sample was observed under a microscope to find burrs around
the holes and cracks between the holes. The mark "good" means that no
burrs or cracks were found, while the mark "poor" means that the sample
showed the formation of burrs and/or cracks.
For comparison, green sheets were prepared in the same manner as described
above except that water was used in place of an organic solvent in wet
grinding. The amount of water used was the same as the amount of the
organic solvent, i.e., the same weight as the total weight of the coarse
powders to be ground.
The following Table 1 shows the test results along with the solvent used in
wet grinding and the specific surface area of the powder mixture after wet
grinding.
TABLE 1
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Organic Specific
Solvent Surface Properties of Green Sheets
Run for Wet Area T. S. % Elon-
Punch-
No. Grinding (m.sup.2 /g)
(kgf/mm.sup.2)
gation
ability
______________________________________
1 Xylene 4.0 0.28 33.8 Good
2 Toluene 4.1 0.30 33.2 Good
3 EBe 3.9 0.28 33.7 Good
4 MEK 4.0 0.29 33.8 Good
5 DEK 4.0 0.30 33.2 Good
6 MIBK 4.1 0.30 33.5 Good
7 EAc 4.0 0.33 31.2 Good
8 IPAc 4.0 0.31 33.2 Good
9 BuAc 4.0 0.31 33.1 Good
10 Xylene + 4.0 0.32 32.6 Good
Toluene*
11 Water 34.4 0.63 9.6 Poor
______________________________________
(Notes)
*Volume ratio of 1:1
Ebe = Ethylbenzene, MEK = Methyl ethyl ketone, DEK = Diethyl ketone, MIBK
= Methyl isobutyl ketone, EAc = Ethyl acetate, IPAc = Isopropyl acetate
BuAc = Butyl acetate
As can be seen from Table 1, in all the runs according to the present
invention (Runs Nos. 1 to 10), the wet-ground powder mixture had a
specific surface area of around 4.0 m.sup.2 /g and the tensile strength
and elongation of the green sheets were good and well-balanced. The
punchability of the green sheets was also good.
In contrast, in Run No. 11 where wet grinding was performed in water, the
specific surface area of the wet-ground powder mixture was increased to
34.4 m.sup.2 /g. As a result, the green sheet had a markedly low
elongation although its tensile strength was high, and hence the
punchability of the sheet was poor.
It is to be understood that although the wet-ground powder mixture was
completely dried by heating in this example in order to determine the
specific surface area, such drying is not necessary in the preparation of
the slurry to cast into green sheets. Similar results to those shown in
Table 1 will be obtained when the slurry is prepared from the wet-ground
powder mixture without drying.
Example 2
The coarse raw powder material used in this example consisted of the same
powders as in Example 1.
A ball mill was charged with 100 parts of the glass powder, 25 parts of the
alumina powder, and 7 parts of the cordierite powder, and 70 parts of an
organic solvent comprised of an alcohol (n-butanol, propanol, or
n-pentanol) and xylene were added to the ball mill along with up to 10
parts of an organic binder and 1 part of DOP. The organic binder used was
either an acrylic resin (polymethyl methacrylate) or a butyral resin
(polyvinyl butyral). After 400 parts of alumina balls (15 mm in diameter)
were added, wet grinding was performed for about 48 to about 100 hours
until the powder mixture had an average particle diameter of 2 .mu.m.
Since wet grinding was performed in the presence of an organic binder in
this example, the resulting wet-ground powder mixture had particles
covered with the organic binder and it was impossible to determine the
specific surface area of the powder mixture.
To the resulting wet-ground mixture from which the alumina balls had been
removed, 1.0 part of a dispersant (acrylic acid oligomer) and an
additional amount of the same organic binder (acrylic resin or butyral
resin) as used in wet grinding were added. The total amount of the organic
binder present in the mixture was 15 parts for each 100 parts of the
powder mixture. The mixture was thoroughly dispersed for 12 hours to form
a slurry. The slurry was cast into sheets using a doctor blade and the wet
sheets were dried at 80.degree. C. to form 200 .mu.m-thick glass ceramic
green sheets, which had good elongation and punchability comparable to the
values shown in Table 1.
The green sheets were fired in air at 900.degree. C. to form sintered glass
ceramic substrates, which were visually observed to determine whether they
turned gray or not. The following Table 2 (binder: butyral resin) and
Table 3 (binder: acrylic resin) show the alcohol used in wet grinding and
the proportion thereof in the entire organic solvent as well as the amount
of organic binder added prior to wet grinding and the presence or absence
of gray color in sintered substrates.
For comparison, the above procedure was repeated except that wet grinding
was performed in the absence of an organic binder.
TABLE 2
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Butyral
Solvent for Resin Color of
Run Wet Grinding Added Sintered
No. Alcohol Wt %* (Parts)
Substrate
______________________________________
1 Butanol 20 0 Gray
2 Butanol 50 0 Gray
3 Butanol 80 0 Gray
4 Butanol 20 3 White
5 Butanol 50 3 White
6 Butanol 80 3 White
7 Butanol 20 5 White
8 Butanol 50 5 White
9 Butanol 80 5 White
10 Butanol 20 10 White
11 Butanol 50 10 White
12 Butanol 80 10 White
13 Propanol 20 3 White
14 Propanol 50 3 White
15 Propanol 80 3 White
16 Propanol 20 5 White
17 Propanol 50 5 White
18 Propanol 80 5 White
19 Propanol 20 10 White
20 Propanol 50 10 White
21 Propanol 80 10 White
22 Pentanol 20 3 White
23 Pentanol 50 3 White
24 Pentanol 80 3 White
25 Pentanol 20 5 White
26 Pentanol 50 5 White
27 Pentanol 80 5 White
28 Pentanol 20 10 White
29 Pentanol 50 10 White
30 Pentanol 80 10 White
______________________________________
(Note) *Remainder: Xylene
TABLE 3
______________________________________
Acrylic
Solvent for Resin Color of
Run Wet Grinding Added Sintered
No. Alcohol Wt %* (Parts)
Substrate
______________________________________
31 Butanol 20 0 Gray
32 Butanol 50 0 Gray
33 Butanol 20 3 White
34 Butanol 50 3 White
35 Butanol 20 5 White
36 Butanol 50 5 White
37 Butanol 20 10 White
38 Butanol 50 10 White
39 Propanol 20 3 White
40 Propanol 50 3 White
41 Propanol 20 5 White
42 Propanol 50 5 White
43 Propanol 20 10 White
44 Propanol 50 10 White
45 Pentanol 20 3 White
46 Pentanol 50 3 White
47 Pentanol 20 5 White
48 Pentanol 50 5 White
49 Pentanol 20 10 White
50 Pentanol 50 10 White
______________________________________
(Note)* Remainder: Xylene
As can be seen from Tables 2 and 3, when the organic solvent used in wet
grinding contained an alcohol, wet grinding in the absence of an organic
binder resulted in the formation of gray-colored sintered glass ceramic
substrates after firing. However, the presence of a small amount of an
organic binder in the alcohol-containing organic solvent used in wet
grinding could avoid such coloration of the sintered substrate.
It will be appreciated by those skilled in the art that numerous variations
and modifications may be made to the invention as described above with
respect to specific embodiments without departing from the spirit or scope
of the invention as broadly described.
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